My 90 G60 with ABS brakes and rear disc brakes has an intermittent problem with the brake pedal getting real hard. Sometimes it does it for a long time and sometimes off and on during the day. currently it's been like this for six months or so. It turns out that when the brakes get hard the rear brakes quit working at the same time but start working again when the pedal gets soft again. I've also noted that when the brake pedal get hard the fluid level in the reservoir gets higher than it's supposed to be but it goes back to the correct level when the pedal gets soft again.
I've replaced the brake mechanism with the push rod (on the real axle I believe) after I noticed that it looked like it had been rusted/locked up perhaps for a while. At first it didn't help but within a week the brakes started working correctly (maybe it took a while for fluid to get up inside the part), but within a month the pedal was back to being intermittently hard again. So it probably wasn't the initial problem, although after not moving for a while it probably rusted in place. It's probably started rusting again in the position that it's in as it or the rear brakes have not worked in the last six months.
Of course the local shop would want way too much to fix it (if they even could after spending many hours on it) but one of the mechanics said he suspects it's nothing too major or expensive, but all the same they don't know much if anything about VW ABS.
After looking at my Bentley manual I see a part in the exploded view that there's a part called a pressure regulator valve (part no. 20 of an exploded view on page 45.7) .
Does anyone know if this part could cause the problems that I've described? Or maybe these exact problems have happened to other Corrado owners and it turned out to be a different part, either way I can sure use some help. All help will be appreciated.